Human fetal liver contains both zinc- and copper-rich forms of metallothionein

Metallothionein was purified under anaerobic conditions from livers of human fetuses ranging from 19 weeks gestational age to term. Homogeneous metallothionein obtained in the absence of reducing agents from the soluble fraction of the tissue which contained 24% and 85% of the total liver copper and zinc, respectively, had less than 1 g atom of copper and about 3.1 g atoms of zinc/mol. Extraction with 1% 2-mercaptoethanol of the insoluble fraction of the tissue (containing 76% and 15% of the liver copper and zinc, respectively) yielded a metallothionein with approximately 2.5 g atoms of copper and 1.3 g atoms of zinc. When the whole tissue was extracted similarly by the procedure of Rydén and Deutsch ((1978) J. Biol. Chem. 253, 519), nearly equal proportions of the two metals were obtained in the soluble fraction from which metallothionein, with about 3.0 g atoms of zinc and 1.5 g atoms of copper, was purified. Thus, zinc- and copper-rich forms of metallothionein are differently distributed between the soluble and insoluble fractions of fetal liver. Nevertheless, the predominant metal in metallothionein of human fetal liver is zinc as is the case in the adult; significant amounts of copper are also present, consistent with the elevated quantity of this metal in the fetal tissue relative to the adult. Cadmium was not detected. Zinc is much more easily removed from the protein than is copper.